Slow wave structures



Aug. 21, 1962 G. M. BRANCH, JR

SLOW WAVE STRUCTURES Filed Jan; 12, 1955 2 Sheets-Sheet 1.

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In ventor Gar/and M. Branch His Attorney Aug.21,1962

G. M. BRANCH, JR

SLOW WAVE STRUCTURES 2 Sheets-Sheet 2 Filed Jan. 12, 1955 Fig.9.

n &7 r n o e wi M n J a a w a N H 6 a UV 6 MJ United States Patent M3,050,657 SLOW WAVE STRUCTURES Garland M. Branch, Jr., Schenectady,N.Y., assignor to General Electric Company, a corporation of New YorkFiled Jan. 12, 1955, Ser. No. 481,450 11 Claims. (Cl. 315-3.6)

This invention relates to helical slow wave structures and, while thisinvention may be incorporated in a large number of diiferent types ofapparatus, it is, by way of example, particularly described inconnection with traveling wave interaction devices generally identifiedas traveling wave tubes.

In a traveling Wave tube utilizing a helix slow wave structure, anelectromagnetic Wave is caused to follow the turns of the helix so as toresult in a reduced velocity of electromagnetic wave propagation alongthe helix axis. An electron beam is caused to travel parallel to theaxis of the helix and at a velocity so that interaction will take placebetween the electromagnetic Wave and the electrons in the electron beam.Customarily, where it is desired to amplify the electromagnetic waveenergy, the electrons in the electron beam have an average velocitywhich is greater than the velocity of the electromagnetic wave along thehelix axis so that energy is transferred from the electron beam to theelectromagnetic wave.

The resulting transfer of energy from electrons in the electron beam tothe electromagnetic wave results in a decrease in the electron beamenergy and a consequent velocity and density modulation of the electronbeam. In order to effect maximum energy transfer from the electron beamto the traveling wave, it would be desirable to be able to control theelectron beam velocity at various points along the helix by applyingdifferent direct current potentials to discrete regions of the helix. Inconventional helix traveling wave tubes this is not possible since thehelix is highly conductive.

A solution to this problem which has been advanced, consists of breakingthe helix up into a number of sections and coupling energy from one ofthe helices to a second section or helix by means of the velocity anddensity modulation of the electrons in the electron beam oralternatively to use two electron beams, one as an energy supplying beamand the other as an electromagnetic wave coupling beam.

Systems such as those above-described tend to be cum bersome andrelatively inefiicient; however, slow wave structures of the typehereinafter described as part of this invention provided a convenientmeans of transferring electromagnetic wave energy between separatehelices which can then be maintained at different potentials. Therefore,in accordance with this invention it is possible to maintain the beampotential at an optimum value and at the same time effectively andefiiciently transfer electromagnetic wave energy between the helixsections.

In addition to being able to maintain the electron beam at an optimumvelocity it is possible in the practice of this invention to effect asubstantially field free drift region between the helices so thatelectron bunches formed as a result of interaction of theelectromagnetic wave with the electrons in the beam can drift and thenrecombine in proper phase with a second section of helix to furtherincrease the energy transfer from the electron beam to theelectromagnetic wave on the slow wave structure.

Another characteristic difficulty in the construction and operation oftraveling wave tubes is that of stabilizing the tube by attenuatingundesired frequency components and backward traveling waves at theoperating frequency. In accordance with another aspect of this inventionthe attenuating structure is formed on an external portion 3,050,657,Patented Aug. 21, 1962 ICC of the vacuum enclosure of the traveling wavetube and therefore can be easily cooled and adjusted for optimumoperation of the traveling wave tube.

It is therefore an important object of this invention to provideimproved slow wave structures.

Another object of this invention is to provide improved slow wavestructures for use in traveling wave interaction devices wherein theelectron beam velocity can be easily and conveniently controlledthroughout the interaction region to effect optimum operation of thedevice.

Another object of this invention is to provide an improved slow wavestructure for use in a traveling wave interaction device wherein thecombined efiects of electron bunching and electron interaction with atraveling electromagnetic wave are easily and conveniently effected.

It is also an object of this invention to provide a slow wave structurehaving in corporated therein stabilization means having high heatdissipating characteristics and convenient reproducibility.

It is also an object of this invention to provide a slow wave structurefor use in a traveling wave interaction device which has a substantiallyfield free drift region the effective length of which can be easily andconveniently varied.

In accordance with an important aspect of this invention there isprovided a slow wave structure comprising a plurality of helices.Between each of these helices there is provided at least one couplinghelix which is oriented to transfer electromagnetic wave energy betweeneach of said plurality of helices.

Other important objects and aspects of this invention will becomeapparent from the following specification and claims when taken inconnection with the figures of the drawing wherein FIG. 1 illustrates anexample of a traveling wave interaction device incorporating thisinvention; FIGS. 2 through 6 illustrate diagrams useful in explainingthe theory of operation of this invention; and FIGS. 7 through 9illustrate examples of other embodiments of this invention.

FIG. 1 illustrates a traveling wave interaction device, hereinafterreferred to as a traveling wave tube which includes an electron gun 10consisting of electron emitting cathode 11, accelerating anode 12, andheater 13 which is energized by power supply 14. The heater makeselectrical connection to cathode 11 at junction 15 to provide the lowpotential connection through lead 16 to power supply 17. Electrons fromcathode 11 follow the general beam path 18 and are collected bycollector 19 which is connected by lead 20 to power supply 17, at alower potential point than accelerating anode 12 in order to efiectdeceleration of the electrons in beam 18. Solenoid 21 provides amagnetic field substantially parallel to the beam path to focus the beamalong the desired path from the cathode 11 to the collector 19.

The traveling wave tube is provided with two helices within glass orceramic vacuum enclosure 22. Helix 23 is provided with an input lead 24through which electromagnetic wave energy can be easily coupled. Helix23 is severed at point 25 to provide a gap 26 along the electron beamwhich extends between point 25 and point 27 on helix 28. Helix 28 isprovided with. an output lead 29 from which amplified electromagneticwave energy can be extracted. The accelerating anode and the helix 23are maintained at the same potential through lead 30 which makesadjustable connection to power supply 17 while helix 28 can bemaintained at the same potential, or, as herein illustrated at aslightly higher potential through separate lead 31.

As thus far described, this tube provides substantially no couplingbetween helix 23 and helix 28 for an electromagnetic wave, exceptthrough the relatively inetl'icient means of the electron beam 18. Inorder to transfer a maximum amount of electromagnetic wave energy fromhelix 23 to helix 28 there is provided coupling helix 32. Helix 32 isWound in an opposite sense to helices 23 and 28 and has substantiallythe same helix pitch angle. Helix 32 is further provided with anadjustable potential source 33. In order to attenuate undesiredelectromagnetic wave energy components which tend to render thetraveling wave tube unstable there is provided an attenuating means 34which may, for example, consist of an aquadag coating or a coating ofother high loss material which is applied to the outer surface of thevacuum enclosure 22 and between a portion of the coupling helix 32 andthe electron beam 18. Attenuators 34, which may for example consist ofrods of high loss material are placed between the turns of couplinghelix 32 to attenuate backward traveling wave energy, particularly atthe operating frequency. It will be noted that this attenuator isisolated from the electron beam and therefore is not subject to electronbeam saturation. In addition the attenuator 34' is easily cooled and maybe varied to obtain optimum operation.

The traveling wave tube illustrated in FIG. '1 is operated by applyingthe necessary operating potentials so as to effect an electron beamflowing between cathode 11 and collector 19 wherein the average electronvelocity is slightly greater than the velocity of an electromagneticwave propagated along helix 13. The manner of computing these velocitiesand designing a helix conductor with the proper pitch to achieve maximumefiiciency and optimum electron beam coupling is Well known in the art.

An electromagnetic wave is applied to input lead 24 land is caused to bepropagated along helix 23. The wave energy interacts with the electronsinthe electron beam to absorb energy therefrom. When the electromagneticwave energy reaches the region where the coupling helix overlaps theinput helix 23 electromagnetic wave energy is induced into the couplinghelix. As will be hereinafter described if the overlap between couplinghelix 32 and input 'helix 23 is one-quarter or an odd number of quarterspace beat wave lengths of the coupled electromagnetic energy, therewill be a complete transfer of the electromagnetic wave energy on helix23 to the coupling helix 32 so that at point 25 there is substantiallyno electromagnetic wave energy on helix 23 since all of this energy hasbeen transferred to coupling helix 32.

In a like manner the electromagnetic wave energy is propagated alongcoupling helix 32 and is subsequently transferred to output helix 28where it further interacts with the electrons in the electron beam sothat an enhanced output is obtained from output lead 29.

It is apparent, then, that there is provided a means for transferringelectromagnetic wave energy from a first helix to a second helix withoutforming any direct current connection therebetween and in such a mannerthat the severed section of the helices do not have to be speciallyterminated and can be operated at the correct direct current potentialto obtain optimum operation of the traveling wave tube.

Before describing the additional features of this invention, it isconsidered desirable in order to obtain a corm plete understandingthereof to discuss the phenomena which occurs when energy is transferredbetween the helices. FIG. 2 illustrates two conducting lines which forpurposes of this discussion may be considered representative of one ofthe helices within the vacuum enclosure and of the coupling helix,respectively. For example, line 1 can be considered to represent helix23 and line 2 to represent helix 32. Curve 35 illustrates the manner inwhich electromagnetic wave energy or power is transferred between thehelices when they are properly oriented and so spaced as to effect apower transfer therebetween.

It will be noted that substantially all of the energy,

4 under idealized conditions is transferred from one line to the otherline every quarter of a space heat wave length Thus it may be seen thata space beat wave length, which will be further described in subsequentparagraphs, amounts to two complete cycles of power transfer betweenlines 1 and 2.

The manner of power transfer will become more apparent from aconsideration of FIG. 3. It is well known that if coupling existsbetween two transmission lines, such that electromagnetic wave energytraveling in one of them induces an electromagnetic wave in the otherline that travels in the same direction, the power originally fed to oneof the lines will gradually transfer to the other. Then the reverseprocess starts, i.e. the power tends to transfer back to the originalline as has been described in connection with FIG. 2. The tworequirements are that the individual transmission lines havesubstantially the same velocities of propagation and that the couplingprovides a forward traveling wave.

FIG. 3 illustrates the normal coupling between two conductors ofdifferent transmission lines wherein there is shown the electric andmagnetic vectors E and H and the resulting Poynting vector S whichdetermines the direction of wave energy propagation along thetransmission line. The differential induced electric and magnetic fieldsdB and dH over the distance dZ are shown. Here the resulting wavetravels in the opposite direction as shown by dS so that the couplingillustrated in FIG. 3 does not result in what is generally termedspacial beating and from which the term beat wave length is derived.

Thus, if two helices are wound in the same direction there is relativelyloose coupling therebetween and substantially no energy is transferredtherebetween; however, if a pair of concentric helices are wound inopposite senses as illustrated in FIG. 4, spacial beating does occur andas a result of the relatively strong coupling therebetween there is ahighly eflicient energy transfer.

In FIG. 4 it will be noted that a wave impressed on the outer helix,line 1, travels down and to the right. Where the pitch angle isrelatively small, a wave is induced on the inner helix, line 2, thattravels up but again progressing toward the right. This backwardcoupling over an incremental distance, together with forward coupling inthe overall structure results in spacial beating and strong couplingbetween the oppositely wound helices. Thus, it may be seen that it ispossible to exchange power between the two helices and that the innerhelix may be the traveling wave tube helix in the vacuum enclosure whilethe second or coupling helix may readily be oriented outside of thevacuum enclosure.

FIG. 5 illustrates the instantaneous amplitudes of the electromagneticwaves on a pair of coupled helices. The coupled wave is always degreesout of phase with the induced wave, as is well known, so that thesecondary effect of the induced wave coupled back to the first helixgives a degree phase shift to subtract power from the original wave.

FIG. 5 illustrates the beat wave envelopes 36 and the instantaneouswaves 37 and 38. From these illustrations it is apparent that underidealized conditions there is a transfer of electromagnetic energy fromone helix to the other helix every one-quarter of a space beatwavelength so that, referring back to the illustration of FIG. 1, ifthere is an overlap of one-quarter or an odd number of quarterwavelengths between coupling helix 32 and input helix 23 there will be,under idealized conditions, no power on helix 23 at point 25 so that nospecial termination is necessary and there will be maximum power oncoupling helix 32. In a like manner, no electromagnetic energy atterminal 39 of helix 32 and maximum energy on output helix 28.

In the foregoing discussion of FIGS. 2 to 5 the description has beenqualified with the statement that these relations apply under idealizedconditions. It should be clearly understood that the effective transferof electromagnetic wave energy over broad bands is possible in anyproperly designed system and that the losses introduced, for example, byhelix resistance, space charge effects and the effects peculiar to thematerial and form. of the vacuum envelope, merely change the well knowndesign parameters and do not alter the fundamental concept of being ableto transfer electromagnetic energy be tween helices which are otherwiseinsulated for direct currents and voltages.

FIG. 6 further illustrates the particular phenomena accompanying thisform of broad band coupling and, specifically, it is a plot of theamplitude of the axial component of the electric field associated with atraveling electromagnetic wave. FIG. 6a illustrates a plot of the twoenergy modes which are characteristic of two coaxial helices wound inthe opposite sense wherein there is strong cross coupling therebetween.The inner and outer helices are diagrammatically indicated by thecircles located along the Zero axis line and curve 40 illustrates onemode of energy propagation and curve 41 illustrates a second mode ofenergy propagation.

Modes 40 and 41 are propagated along the helices in a directionsubstantially perpendicular to the surface of the drawing and atdifferent velocities. Mode 40 is characterized by in-phase currents ofapproximately equal amplitudes in the two helices and mode 41 bysubstantially 180 degree out-of-phase currents. Thus, for example, ifone helix, say the inner helix 28, is terminated by an open circuit suchas open circuit at point 27, so that no current can flow in helix 28 atpoint 27, an approximately equal mixture of the two modes is excitedwhen an electromagnetic wave is propagated in the outer helix, forexample, coupling helix 32.

It is interesting to note that the conditions prevalent at point 27 andthroughout the gap 26 in FIG. 1 are generally illustrated in FIG. 6b bycurve 42 which illustrates the distribution of electromagnetic fieldacross the tube when substantially all of the electromagnetic waveenergy is on the outer helix. It will be seen that under theseconditions there is a very strong electromagnetic field between thehelices and having a peak centered over the outer coupling helix 32 andthat there is substan tially no electromagnetic field in the region 43between the wires of the inner helices. This intermediate region 43 isthe region through which the electron beam passes and the distributionof the electromagnetic fields immediately suggests that there isestablished a substantially field free drift region in gap 26 which isfurther enhanced by the shielding eflect of the high loss coating 34.Electrons in the beam can drift through this field free region and bunchin accordance with the velocities imparted thereto by the interaction ofthese electrons with the electromagnetic wave on helix 23.

Again considering FIG. 6, it is noted that the two modes are thenpropagated down the coaxial structure until an elapsed phase angle ofone of the modes is 180 degrees greater than that of the other mode atwhich point the two modes now interfere to effectively cancel thecurrents on the outer helix as illustrated by curve 44 in FIG. 6b. Thus,if the outer helix is discontinued at this point the wave propagatesalong the inner helix alone and the radial frequency or electromagneticenergy has been efiectively transferred from the outer helix to theinner helix.

A consideration of the energy distribution in curve 44 of FIG. 6bimmediately suggests that substantially all of the energy on the helicesis within the vacuum enclosure so that there is strong interactionbetween the electron beam and the electromagnetic wave energy propagatedalong the inner helix.

In view of the foregoing it is readily apparent that the above mentionedteachings can be applied to the traveling wave tube structureillustrated in FIG. 1 to effect the objects of this invention. It isnoted, and it can easily be shown that coaxial helix couplers introducevery small reflections over extremely wide bandwidths such as, forexample, frequency ranges in the order of 5 to l, Where such wide bandacceptance is desired. For example, a traveling wave tube of the typeillustrated in FIG. 1 can readily be designed to operate efliciently andeffectively over a frequency range from approximately 300 to 1500megacycles.

The attenuator 34 illustrated in FIG. 1 as has been previouslymentioned, may consist of any lossy material such as an aquadag coatingwhich can be easily and conveniently sprayed on the outer surface of thevacuum enclosure 22. Thus, by placing theattenuator external to thevacuum enclosure, the amount of attenuation can be easily controlled, ismore easily cooled, is relatively free from saturation effects due tothe action of the electron beam, and furthermore is in a strongelectromagnetic field as a result of substantially all of the powerbeing transferred to coupling helix 32. Aquadag coating 34 extends asmall distance over the regions where the inner helices are terminated;however, it does not extend beyond the outer ends of the coupling helixsince to do so would result in ineflicient transfer of power to thecoupling helix and from the coupling helix back to the inner helix.Thus, attenuator 34 stabilizes the tube against regeneration andoscillation by damping spurious oscillations and the electromagneticwaves associated with these oscillations which are reflected from theoutput termination or load and propagated back along the slow wavestructure of the tube input.

Very often, the reflected electromagnetic waves at the operatingfrequency are at such a high power level that it is necessary to provideadditional attenuation such as lossy rods or members 34 which are hereinshown as being placed between the turns of the coupling helix 32.Substantially all of the electromagnetic wave energy at the operatingfrequency passes through the coupling helix and therefore any reflectedwave energy, which would tend to render the interaction device unstable,can be absorbed by this easily cooled and controlled attenuating means.

As has been previously mentioned the quarter space beat wave lengthoverlapping section of coaxial crosswound double helices provides a verywide-band reflectionless transition between the inner helix 23 and thecoupling helix 32 in a relatively short physical distance. The innerhelix, which is effectively severed, can have its input and outputsections, 23 and 28, respectively, operated at different voltages toprovide optimum gain and/ or efliciency without the customary necessityof introducing long tapered attenuators near the severed ends. Suchattenuators interfere with the interaction between the slow wave on theinner helices and the electron beam and reduce the efiiciency as well asthe gain of the tube.

It is apparent, from FIGURE 6, that by increasing the diameter of theouter or coupling helix the field strength in the proximity of theelectron beam due to the electromagnetic wave energy on the couplinghelix will decrease. If it is desired to design a relatively narrow bandpass coupling helix it is only necessary to utilize a coupling helixhaving a relatively large diameter compared to the diameter of the innerhelices.

As has been previously mentioned, in connection with the discussion ofthe curves in FIG. 6 of the drawing, it is theoretically possible totransfer all of the electromagnetic wave energy from the inner helix tothe coupling helix such that there is substantially no electromagneticfield in the vicinity of the electron beam and consequently theelectromagnetic wave is completely decoupled from the electron beam.Under ideal conditions an outer helix which has a diameter approximatelyone and one-half times the diameter of the inner helix is suflicient todecouple completely the traveling wave from the beam.

Thus, the attenuator inside the coupling helix and outside of the innerhelix gap 26 as well as the greater diameter and lower axial impedanceof coupling helix 32, reduces the direct coupling between the beam andthe outer helix so that a substantially field free drift region 26 isprovided between the two helices 23 and 28. Thus electrons which haveinteracted with the electromagnetic wave on helix 23 are permitted todrift through region 26 and bunch or group in accordance with thevelocities imparted thereto by the electromagnetic Wave. That is, thoseelectrons moving slower than the actual velocity of the electromagneticwave are accelerated and those moving faster than the electromagneticwave are decelerated so that there is a grouping or bunching in the gapor drift space 26.

If the drift region 26 is made sufi'lciently long and helix 28 is of theproper length the electron bunches will have completely formed and willinteract in proper phase with the electromagnetic wave which istransferred from the coupling helix 32 back to the helix 28 so as toresult in a considerably enhanced electromagnetic wave output from lead29 as a result of this combined effect. The length of the drift region26 can be initially adjusted so as to effect the proper spacing betweenthe helices 23 and 28 and, in addition, power supply 33 is provided sothat the potential of the coupling helix can be varied and the effectivelength of the drift region controlled for optimum bunching and phasingof these bunches with the electromagnetic wave. What has just beendescribed then, amounts to recombining the density modulated beam at theend of the drift space 26 in proper phase with the electromagnetic waveenergy transmitted through the coupling helix so as to result inenhancement of the efficiency and gain of the tube.

Under certain conditions it may be necessary or desirable to have arelatively small gap between the ends of the helices 23 and 28. If thisis the case, the necessary attenuation can be obtained by applyingaqua-dag coating to other regions of the outer surface of vacuumenclosure 22 and using a short section of coupling helix to transfersome of the wave energy to the outer surface so as to provide a strongelectromagnetic field in the region of the attenuating means.Alternatively a coupling helix of lossy material can be used whichcouples some of the energy to the outer helix from one or more of theinner helices but in which there is no physical inner helix gap. It isalso readily apparent that Where it is desirable a relatively longtraveling wave tube having a number of such drift regions can beprovided so that the tube will in effect consist of a plurality of driftregions 26 with associated coupling helices 32.

FIG. 7 illustrates an alternative embodiment of this invention whereinelectromagnetic wave energy is introduced to inner helix 46 throughcoupling helix 45 and is extracted from inner helix 47 by means ofcoupling helix 48. Coupling between helices 46 and 47 is elfected bymeans of outer coupling helix 49. This embodiment is provided withstabilizing aquadag coating 50 and an attenuator 51. Since the couplinghelix 49 is somewhat remote from drift region generally indicated by 52,under some conditions it may be desirable to provide a drift tube in thegap between helices 46 and 47. This drift tube 53 consists of aconducting tubular member through which the electron beam can pass sothat it will be in a completely field free region. In addition there isprovided a tap '54 so that the drift tube 53' can be maintained at thedesired potential and so that the effective length of the drift tube canbe conveniently varied. It is apparent that this drift tube can be usedin combination with any or all of the embodiments previously shown anddescribed in connection with the illustration of FIG. 1 and that theaccompanying electron beam forming means, magnetic field and potentialsupplies are not shown in FIG. 7 merely as an aid in reducing thecomplexity of this description.

Thus, it is apparent that the field free drift region and therefore thedrift time can be varied by varying the length of the gap, the length ofdrift tube 53 and/or the potentials applied to the coupling helix or thedrift tube or both. Energy is transferred to the helix 46 and from thehelix 47 by the same mechanism as hereinbefore described in connectionwith the transfer of energy from an inner helix to an outer helix. Theadvantages of this method of coupling are readily apparent in that theyreduce the number of glass-to-metal seals that must be formed in orderto extract energy from the inner helix and require less complicatedterminating structures at the ends of the helices within the vacuumenclosure.

FIG. 8 illustrates an alternative construction that can be utilized toeffect elficient attenuation of undesired modes and to obtain somedegree of electron bunching. In FIG. 8 there is shown input helixsection 55 and output helix section 56 separated by lossy helix section57 so that there is an effective gap for radio frequency currents in theintermediate region defined by the lossy section of helix 57. The lossysection 57 can be formed, for example, by applying a high loss materialto a wire helix. Effective radio frequency electromagnetic wave energycoupling between helices 55 and 56 is effected by coupling helix 58 andsome stabilization is provided by the lossy section. An attenuator forthe operating frequency reflected electromagnetic wave energy can beplaced in juxtaposition to the helix 58 and therefore be in a strongelectromagnetic wave energy region and yet be easily cooled and variedfor optimum tube operation. It is apparent that the structure hereinillustrated is not suited for operating conditions where it is desiredto operate the input helix section and the output helix section atdifferent potentials since lossy helix section 57 provides a relativelygood direct current path between sections 55 and 56.

FIG. 9 illustrates a plurality of helix sections 59, 60 and 61 which arecoupled by coupling helix sections 62 and 63. Potential leads 64, 65 and66 are coupled to helices 5'9, 60 and 61, respectively, and provide ameans for maintaining these helix sections at the proper potential foroptimum operation of a traveling wave tube. It is apparent that thismultiple helix structure can be provided with any one or all of theother features hereintofore described in order to provide a multigapstructure having substantially field free drift regions between therespective inner helices. Thus, there is provided an effectivelyperiodic traveling wave tube structure wherein there are providedseveral beam voltage steps to maximize the tube efficiency.

In view of the foregoing, it is readily apparent that among theadjustable parameters, whose optimum values can be determined by theoryor easy experimentation, include the length and pitch of the inputhelix, length and pitch and relative diameter of the coupling helixincluding the length of the drift section and the length and pitch ofthe output helix. Thus, it is possible to design an efficient andeffective traveling wave tube incorporating this invention which can beoperated over a wide frequency band or a relatively limited frequencyband depending on the desired service and the band previously mentionedmay be placed anywhere within a large portion of the radio frequencyspectrum. In addition, it is apparent that this invention can be appliedto interaction devices using low density annular electron beams and toany of the many other applications of slow wave structures.

In view of the foregoing, it is readily apparent that this invention issubject to a large number of modifications and variations and that theexamples herein described are considered to be representative only.Therefore, it is intended to include in the appended claims all suchmodifications and variations as come within the true spirit and scope ofthis invention.

What I intend to claim and protect by Letters Patent of the UnitedStates is:

1. In a traveling wave interaction device including means for producinga beam of electrons within a vacuum enclosure, a slow wave structurecomprising first and second helical sections oriented within saidenclosure and in energy transferring relationship with the beam ofelectrons, the adjacent ends of said helical sections being in spaced,longitudinal relation with respect to the propagation of high frequencywaves, a coupling helix placed outside the vacuum enclosure and orientedin overlapping relation with the adjacent ends of said helical sectionsto transfer electromagnetic wave energy between said helical sections,said coupling helix having a pitch substantially equal to the pitch ofsaid helical sections but opposite in direction, a first attenuatororiented between the coupling helix and the interior of the vacuumenclosure and a second attenuator coupled to the coupling helix tostabilize the interaction device, whereby heat energy in the attenuatorsis easily dissipated and the amount of attenuation is easily controlledto effect idealized operation of the interaction device.

2. In a traveling wave interaction device including means for producinga beam of electrons, a slow wave structure comprising first and secondhelical sections oriented in energy transferring relationship with theelectron beam to define a gap between the helices along the electronbeam, a coupling helix overlapping the adjacent ends of the first andsecond helical sections to transfer electromagnetic wave energy acrossthe gap and between the helical sections, said coupling helix having apitch substantially equal to the pitch of said first and second helicalsections but opposite in direction whereby the electrons crossing saidgap are in a substantially field free region and can bunch in accordancewith velocities imparted to the electrons as a result of electroninteraction and with electromagnetic wave energy on said first helicalsection, said gap being of proper length so that said bunches interactin phase with electromagnetic wave energy transferred to said secondhelical section by the coupling helix whereby the gain and efficiency ofsaid interaction device are enhanced.

3. In a traveling wave interaction device including means for producinga beam of electrons, a slow wave structure of the type defined by claim2 wherein a conductive member is oriented in said gap and in proximityto said electron beam to effect a more completely field free driftregion within the gap in the interaction device.

4. In a traveling wave interaction device including means for producinga beam of electrons, a slow wave structure of the type defined by claim3 wherein a potential is applied to the conductive member to control theeffective electrical length of the field free drift region.

5. In a traveling wave interaction device including means for producinga beam of electrons, at slow wave structure comprising a first helix anda second helix oriented in energy transferring relationship with theelectron beam to define a gap between the helices and along the electronbeam, a coupling helix overlapping the adjacent ends of said first andsecond helices to transfer electromagnetic wave energy across the gapand between the helices, said coupling helix having a pitchsubstantially equal in magnitude to the pitch of said first and secondhelices but opposite in direction, a lossy material surround ing saidgap and oriented between said coupling helix and the electron beamwhereby beam electrons crossing said gap are in a substantially fieldfree drift region and can bunch in accordance with the velocitiesimparted to the electrons as a result of interaction of the electronswith the electromagnetic wave energy on said first helix, said gap beingof proper length so that said electron bunches interact in proper phasewith the electromagnetic wave energy transferred to said second helix bythe coupling helix whereby the gain and efficiency of said interactiondevice are enhanced.

6. In a traveling wave interaction device including means for producinga beam of electrons, a slow wave structure comprising a first helix anda second helix oriented in energy transferring relationship with theelectron beam to define a gap between the helices and along the electronbeam, means applying electromagnetic wave energy to said first helix andmeans for extracting electromagnetic wave energy from said second helix,a coupling helix overlapping at least a portion of the adjacent ends ofsaid first and second helices to transfer electromagnetic wave energyacross the gap and between the helices, said coupling helix having apitch substantially equal in magnitude to the pitch of said first andsecond helices but opposite in direction, a lossy material surroundingsaid gap and oriented between said coupling helix and said electron beamwhereby electrons crossing said gap are in a substantially field freeregion and can bunch in accordance with the velocities imparted to theelectrons as a result of interaction of the electrons with theelectromagnetic wave energy on said first helix, means for applying apotential to said coupling helix to control the effective length of thefield free drift region so that said bunches interact in phase with theelectromagnetic wave energy transferred to said second helix by thecoupling helix to result in enhanced gain and efliciency of saidinteraction device.

7. In a traveling wave interaction device including means for producinga beam of electrons, the combination comprising first and second helicalsections of conducting material positioned in energy exchanging relationwith said beam of electrons and having the adjacent ends thereofpositioned in spaced longitudinal relation with respect to the path ofthe beam and the propagation of high frequency waves and a couplinghelix extending between said helical sections and overlapping theadjacent ends thereof, said coupling helix having substantially the samepitch as said first and second helical sections but being wound in theopposite direction to provide for the transfer of energy between saidfirst and second helical sections through said coupling helix.

8. In a traveling wave interaction device including means for producinga beam of electrons, the combination comprising first and second helicalsections of conducting material positioned in energy exchanging relationwith said beam of electrons and having the adjacent ends thereofpositioned in spaced longitudinal relation with respect to the path ofthe beam and the propagation of high frequency waves and a couplinghelix extending between said helical sections and overlapping theadjacent ends thereof by an amount corresponding substantially to an oddnumber of one-quarter wave lengths at the space heat wave lengths of theelectromagnetic energy to be coupled, said coupling helix havingsubstantially the same pitch as said first and second helical sectionsbut being Wound in the opposite direction to provide for the transfer ofenergy between said first and second helical sections through saidcoupling helix.

9. In a traveling wave interaction device including means for producinga beam of electrons, the combination comprising first and second helicalsections of conducting material positioned in energy exchanging relation with said beam of electrons and having the adjacent ends thereofpositioned in spaced longitudinal relation with respect to the path ofthe beam and the propagation of high frequency waves, a coupling helixextending between said helical sections and overlapping the adjacentends thereof, said coupling helix having substantially the same pitch assaid first and second helical sections but being wound in the oppositedirection to provide for the transfer of energy between said first andsecond helical sections through said coupling helix and meansmaintaining said coupling helix at a direct current potential differentthan the direct current potential of said first and second helicalsections.

10. In a traveling wave interaction device including means for producinga beam of electrons, at slow wave structure comprising first and secondhelical sections, said first and second helical sections being wound ina first direction and oriented in energy transferring relationship withthe beam of electrons and a coupling helix wound in a sense opposite tothat of said first and second helical sections and oriented to transferelectromagnetic wave energy between said first and second helicalsections, said coupling helix having substantially the same pitch assaid first and second helical sections and having a position coaxialWith and overlapping with respect to the adjacent ends of said helicalsections.

11. A traveling-Wave tube amplifier comprising an electron gun includinga cathode maintained at a predetermined reference potential forproducing an electron stream, means for directing said stream along apredetermined path, a collector electrode disposed opposite saidelectron gun to intercept the stream electrons, an input helixmaintained at a first predetermined potential with respect to saidreference potential and disposed about said path adjacent said electrongun for propagating an electromagnetic Wave at a predetermined velocity,said predetermined velocity being small in comparison to the velocity oflight, a principal helix maintained at a second predetermined potentialwith respect to said reference potential and electromagnetically coupledto said input helix and disposed between said input helix and saidcollector electrode for propagating said wave, said first potentialbeing of such a value as to maintain said input helix at adirect-current potential to minimize the coupling loss between both ofsaid helices with regard to the growing wave portion of saidelectromagnetic Wave.

References Cited in the file of this patent UNITED STATES PATENTS2,584,308 Tiley Feb. 5, 1952 2,588,832 Hansell Mar. 11, 1952 2,616,990Knol et al. Nov. 4, 1952 2,623,193 Bruck Dec. 23, 1952 2,636,948 PierceApr. 28, 1953 2,660,689 Touraton et a1 Nov. 24, 1953 2,733,305 DiemerJan. 31, 1956 2,782,339 Nergaard Feb. 19, 1957 2,793,315 Haeif et al.May 21, 1957 2,804,511 Kompfner Aug. 27, 1957 2,806,177 Haeif Sept. 10,1957 2,811,673 Kompfner Oct. 29, 1957 2,814,779 Mendel Nov. 26, 1957FOREIGN PATENTS 668,168 Great Britain Mar. 12, 1952

